This document claims priority and contains subject matter related to Japanese Patent Application No. 2000-272196, filed on Sep. 7, 2000, the entire contents which are incorporated by reference.
1. Field of the Invention
The present invention relates to a method and an apparatus for image formation, and more particularly to a method and an apparatus that can form a high quality image by suppressing an occurrence of toner scatter and a partial omission of a toner image.
2. Discussion of the Background
An image forming apparatus, in which a toner image is formed on a seamless image bearing belt and the toner image is transferred onto a transfer medium by a transfer device, such as a copying machine, a printer, a facsimile, or a multifunctional apparatus having at least the above-described two functions is commonly known. The image bearing member includes, for example, a photoconductive belt on a surface of which a toner image is formed or an intermediate transfer belt onto which the toner image is transferred from a photoconductive element, according to a type of a developing device.
FIG. 10 is a schematic drawing illustrating a construction of a transfer section of a conventional image forming apparatus in which an intermediate transfer belt is used as an image bearing member. As shown, an intermediate transfer belt 1A is spanned around a plurality of rollers including a roller 2A and other rollers (not shown), and is driven in a direction indicated by an arrow xe2x80x9cA.xe2x80x9d Each toner image of different colors is transferred onto the surface of the intermediate transfer belt 1A from a photoconductive element (not shown) one after another while being superimposed on each other. A transfer roller 3A, as an example of a transfer device, is provided at a position opposed to the roller 2A via the intermediate transfer belt 1A.
As illustrated in FIG. 10, the transfer roller 3A press-contacts with the roller 2A via the intermediate transfer belt 1A when a toner image formed on the surface of the intermediate transfer belt 1A is transferred onto a recording medium including a transfer medium 4A. At this time, the transfer roller 3A rotates in a direction indicated by an arrow, and the transfer medium 4A is conveyed in a direction indicated by an arrow xe2x80x9cBxe2x80x9d to pass through a nip 5A. As described above, the transfer roller 3A press-contacts with the roller 2A via the intermediate transfer belt 1A and the transfer medium 4A. At this time, a transfer voltage with a reverse polarity of toner image formed on the surface of the intermediate transfer belt 1A is applied to the transfer roller 3A.
The toner image formed on the surface of the intermediate transfer belt 1A is then transferred onto the surface of the transfer medium 4A which is conveyed in the direction indicated by the arrow xe2x80x9cBxe2x80x9d while contacting the surface of the intermediate transfer belt 1A. The transfer medium 4A, which has been conveyed through the nip 5A formed between the roller 2A and the transfer roller 3A, is conveyed to a fixing device so that the toner image transferred onto the surface thereof is fixed.
As explained above, the transfer medium 4A is conveyed to the nip 5A from an inlet side xe2x80x9cI.xe2x80x9d A wedge-shaped gap xe2x80x9cGxe2x80x9d is formed between the intermediate transfer belt 1A and the transfer medium 4A in a region in the inlet side xe2x80x9cI.xe2x80x9d Further, a portion of the transfer medium 4A, which is placed adjacent to the nip 5, is charged with the reverse polarity of the toner image because the above-described transfer voltage has been applied to the transfer roller 3A. Thus, toner on the surface of the intermediate transfer belt 1A electrostatically flies and adheres to the surface of the transfer medium 4A as indicated by the arrows xe2x80x9cC.xe2x80x9d Such a phenomenon is called xe2x80x9ctoner scatter.xe2x80x9d When toner scatter occurs, the scattered toner appears around the image transferred onto the transfer medium 4A, which has passed through the nip 5A, in a blotted condition resulting in a degradation in the quality of the toner image.
The above-described inconvenience is also caused when the image bearing belt includes a photoconductive belt or a dielectric belt and the transfer medium includes an intermediate transfer element or a recording medium. The intermediate transfer belt 1A generally has a volume resistivity of, for example, 108 xcexa9 cm to 1013 xcexa9 cm. The above-described toner scatter frequently occurs when the intermediate transfer belt 1A is used. A reason for the frequent occurrence of toner scatter is because a force of toner that electrostatically adheres to the surface of the intermediate transfer belt 1A is less than a force of the toner that electrostatically adheres to an insulator. Therefore, the toner adheres to the surface of the intermediate transfer belt 1A in a state that the toner is comparatively easy to move.
Further, toner in an upper-most layer is easily moved when each toner color image is formed on the surface of the intermediate transfer belt 1A while superimposed on each other, because an amount of the toner per unit area on the surface of the intermediate transfer belt 1A is increased. In addition, when the surface of the intermediate transfer belt 1A (on which the toner adheres in the state that it can easily move) is opposed to the charged transfer medium 4A with the minute gap xe2x80x9cGxe2x80x9d therebetween, the toner on the surface of the intermediate transfer belt 1A easily flies onto the surface of the transfer medium 4A electrostatically, resulting in the frequent occurrence of the toner scatter phenomenon.
As illustrated in FIG. 11, a roller 7A is provided to approximately parallelize a portion of the intermediate transfer belt 1A (which is positioned between the roller 2A opposed to the transfer roller 3A and the roller 7A supporting the intermediate transfer belt 1A) with the conveying direction xe2x80x9cBxe2x80x9d of the transfer medium 4A conveyed to the nip 5. With this configuration, the transfer medium 4A starts to contact the surface of the intermediate transfer belt 1A at a position substantially upstream from the nip 5 in a moving direction of the intermediate transfer belt 1A. Thus, when a portion 8A of the transfer medium 4A conveyed to a vicinity of the nip 5A is charged by a voltage applied to the transfer roller 3A, the toner on the surface of the intermediate transfer belt 1A does not fly onto the surface of the portion 8A of the transfer medium 4A or a very small amount of the toner flies thereon. Therefore, an occurrence of the toner scatter is suppressed.
In addition, when the diameter of the rollers 2A and 7A supporting the intermediate transfer belt 1A is too small, the intermediate transfer belt 1A passing over the rollers tends to curl. Thus, an uneven transfer of an image to the transfer medium 4A from the intermediate transfer belt 1A arises, resulting in a degenerated toner image. Further, when the diameter of the roller 2A opposed to the transfer roller 3A is too small, a length of the nip 5A (i.e., the nip width) in the conveying direction of the transfer medium 4A is decreased, resulting in a reduction of a transfer efficiency of the toner image. Therefore, the diameter of rollers 2A and 7A cannot be decreased very much.
In addition, when the diameter of the rollers 2A and 7A is increased, a length xe2x80x9cDxe2x80x9d of the transfer medium 4A, in which the transfer medium 4A contacts the surface of the intermediate transfer belt 1A before the transfer medium 4A reaches the nip 5, is increased. Thus, the transfer medium 4A starts to contact the surface of the intermediate transfer belt 1A at a position substantially upstream from the nip 5 in a moving direction of the transfer medium 4A.
A portion 9A of the transfer medium 4A, which is an upstream side portion of the transfer medium 4A in the conveying direction of the transfer medium 4A, is positioned substantially away from the transfer roller 3A. Therefore, the portion 9A of the transfer medium 4A is not charged by an effect of a voltage applied to the transfer roller 3A. Further, even if the portion 9A of the transfer medium 4A is charged by the effect of the voltage applied to the transfer roller 3A, the potential is very low.
Thus, the portion 9A of the transfer medium 4A does not electrostatically tight-contact the surface of the intermediate transfer belt 1A. Then, the portion 9A of the transfer medium 4A may not be brought into tight-contact with the intermediate transfer belt 1A due to projections and depressions formed on the surface of the intermediate transfer belt 1A or a slack of the intermediate transfer belt 1A caused by a curl given to the intermediate transfer belt 1A. Further, the portion 9A of the transfer medium 4A may slightly deviate from the intermediate transfer belt 1A. In addition, a part of toner image placed between the surface of the intermediate transfer belt 1A and the portion 9A of the transfer medium 4A is then disturbed. By this disturbance, a partial omission of a transferred image may occur (i.e., a concentration of a part of the toner image transferred onto the transfer medium 4A is very low), which degrades the quality of the image.
Further, an image forming apparatus using a transfer device other than a transfer roller is similarly inconvenienced by the above-described inferior image.
Accordingly, an object of the present invention is to solve the above-mentioned and other problems.
Another object of the present invention is to advantageously provide a novel image forming apparatus and method in which an occurrence of toner scatter and a partial omission of a transferred image, which is caused by a disturbance of toner image, is effectively suppressed.
To solve these and other problems, the present invention provides a novel image forming apparatus including an image bearing belt in an endless form configured to be driven while being spanned around a plurality of rollers, and in which a toner image is formed on a surface of the image bearing belt. Also included is a transfer mechanism disposed at a position opposed to a respective one of the plurality of rollers via the image bearing belt and configured to transfer the toner image formed on the surface of the image bearing belt onto a transfer medium conveyed through a region where said transfer device opposes the respective one of the plurality of rollers with the image bearing belt passing therebetween. Further, a transfer voltage with a reverse polarity of the toner image formed on the surface of the image bearing belt is applied to the transfer device when the toner image is transferred onto the transfer medium. In addition, a belt guide member is also fixedly disposed at an upstream side of the region where the transfer mechanism opposes the respective one of the plurality of rollers in a moving direction of the image bearing belt. The belt guide member is configured to press-contact with an underside of the image bearing belt so as to protrude the image bearing belt such that a portion of the image bearing belt is substantially parallel with a conveying direction of the transfer medium. Further, a side surface the belt guide member that press-contacts with the underside of the image bearing belt includes a flocked surface.